The method described in application WO-A-03/048403 A1 is part of a global project intended to reduce the production costs of pre-painted metal strips. In this context, the metallurgists hope to incorporate the lacquering process at the end of the galvanizing line.
The main difficulty to obtain this result has been to find a conversion treatment for the strip that is fast enough to be put between the galvanizing and the painting treatment. The above-mentioned method has also been considered as an alternative to treatments based on chromates.
Being based on the use of the strip's residual heat after galvanizing and spinning, this method does not require any external energy input in order to work.
On the installation side, it is preferably carried out in the descending section that follows the zinc bath. From a practical point of view, it can be installed in place of the tank of dematerialized water that completes the cooling with jets of water steam. The compact deposition system considered here may be a bath or a spray system (wave of water, spraying with jets, etc.). Thus, with the help of some modifications, the investment in the new equipment is limited.
First Approach: Ultra-Fine Layer
Ultra-fine layers, typically less than 100 nm, produced by the proposed method can only be considered for solutions with a low concentration of particles, low strip temperatures or even both. The possibility of also being able to produce deposits of this type for solutions with high concentrations of nanoparticles and/or at high temperature would be very usefully for a simple in-line adaptation of the method.
Moreover, this objective is crucial for obtaining a deposit that perfectly adheres to the metal and for good internal cohesion of the oxide layer. Indeed, for a solution with a low concentration, the nanoparticles in suspension are some distance from each other and thus have little tendency to correctly agglomerate when the solvent evaporates.
However, one problem caused by the use of solutions with medium and high concentration is the formation of localized excessive thicknesses that form a network of very friable “ribs” on the surface of the oxide deposit, as shown in FIG. 1. These result from the preferential precipitation at the interface between the solution and the vapor phase caused during immersion, as diagrammatically described in FIG. 2. This can be seen both on the samples produced in a bath (FIG. 2.a) or by spraying (FIG. 2.b) and it is detrimental to the subsequent adhesion of paint.
Document JP-A-63 072887 teaches a method for producing a steel strip by hot dipping showing excellent resistance to corrosion and good mechanical resistance so that, before the drying of the first layer made of zinc or aluminum, an aqueous solution containing dissolved silica and/or aluminum, lithium silicate, etc. is pulverized on the surface of the strip so as to form an oxide layer comprising SiO2, Al2O3 or Li2SiO, separately or in a mixture. However, a film of chromate is also formed on the oxide layer so as to increase the resistance to corrosion and the adhesion of the oxide layer, in contrast to the method of the previous application WO-A-03/048403, which was free of hexavalent chrome. This shows that good adhesion of the nanoparticles is far from certain.
Document JP-A-62 166667 discloses a method for forming an oxide layer on the surface of a steel strip coated by hot dipping with a layer of Zn or of a Zn—Al alloy with the aim of preventing deep grey discoloration of the strip. A solution containing one or several of the oxides ZrO2, Cr2O3, Al2O3, Y2O3, CeO2, ZrBiO4 and Sb2O3 is pulverized on the strip after immersion and thus its temperature is ≧100° C. at a concentration in the range of 1-100 mg/m2. The water is evaporated by the intense heat of the steel strip, with the formation of the oxide film. A film of chromate is then formed on the above-mentioned oxide layer. It should be noted that a check of the thickness of the layer is neither considered nor described although this is crucial for good adhesion of the deposit. It seems that the layer of chromate is there to compensate for this omission.
Second Approach: Better Stability of the Solution Depending on the Temperature
When the strip is plunged into the bath, it transfers its heat to the colloidal solution. So as to avoid overheating the latter and thus adversely affecting the bath, it is clearly the intention to remove the excess energy by means of external circulation and a heat exchanger. In fact, despite the presence of this equipment, it has been noted that the bath is adversely affected. It seems that the excess heat retained at the metal-solution interface is responsible for this and causes the precipitation of the solution.
So as to be able to guarantee a satisfactory useful life of the bath, it is absolutely necessary to find a method that allows to use the solution right up until the solvent boils.
Third Approach: a Wider Margin for Maneuver
It is possible to adapt the cooling equipment preceding the tank containing the colloidal solution or the banks of sprays so as to be able to guarantee a constant entry temperature over time. It is necessary to control this parameter so as to guarantee constant thickness of the deposit of nanoparticles on the substrate.
However, in order to be competitive relative to a cold strip treatment placed on the same location, apart from the control of the bath, which is common, it would be preferable to be able to dispose of the need for precision in the temperature or to reduce it. Thus, so that it is less of a restriction to the user, this method should be able to function with a relatively high level of uncertainty regarding the temperature level.
Another disadvantage of an “immersion deposit” treatment such as this in comparison with a cold method is that it is, in addition to being affected by a change in the temperature of the substrate, sensitive to a variation in the thickness of the strip. In fact, at a given temperature, for a given material, the quantity of thermal energy stored is a function of the volume of the body, hence of the thickness in the case of a flat product. In fact, on a galvanizing line, steel strips of different thicknesses can be processed.
Aims of the Invention
The present invention aims to provide a method for coating a metal with an ultra-fine protective film of oxide, preferably of silicon, titanium, zirconium, cerium, yttrium or antimony.
An additional aim of the invention is to allow maximum flexibility of the method relative to the entry temperature of the strip into the bath.
Another aim of the invention is to guarantee reproducibility of the deposit in terms of thickness with a light or heavy weight of the layer.
Another aim of the invention is to guarantee a useful life of the solution that meets the metallurgist's requirements.
Main Characteristic Elements of the Invention
A first aspect of the present invention relates to a method for continuously coating a substrate in motion such as a metal strip made of steel, the coating formed being an ultra-fine film of a thickness of between 10 and 100 nm, deposited on the substrate:                from a solution containing nanoparticles of oxides,        in conditions of controlled pH,        said substrate being at a temperature higher than 120° C.,        the total duration of the deposition being less than 5 seconds, preferably less than 1 second,characterised in that at least one chemical additive, called a “refiner”, is incorporated into said solution, said refiner having, mutatis mutandis, the effect of restricting the formation of said coating.        
In the context of the invention, the substrate to be coated is either a bare metal, preferably steel, stainless steel (or “inox”), aluminum, zinc or copper; or a first metal coated with a second metal, preferably a steel strip coated with a layer of zinc, aluminum, tin, or of an alloy of at least two of these metals.
The nanoparticles comprise oxides, preferably SiO2, TiO2, ZrO2, Al2O3, CeO2, Sb2O5, Y2O3, ZnO, SnO2 or mixtures of these oxides, are hydrophilic and/or hydrophobic, have a size of between 1 and 100 nm and are in the solution with a content of between 0.1 and 10%, and preferably between 0.1 and 1%.
The concentration of refiner is between 1 and 20 g per liter (g/L) of solution, preferably between 5 and 10 g/L.
More particularly, the refiner used for the deposit of silica nanoparticles is selected from the group of compounds comprising catechin and its derivatives, hydrofluoric and boric acids, borates, sodium and potassium carbonates and hydrogen carbonates, ammonium hydroxide and amines that are soluble in water. The refiner used for a deposit of nanoparticles of stannous or stannic oxide is selected from the group of compounds comprising borates, potassium carbonates and hydrogen carbonates, ammonium hydroxide and amines that are soluble in water. The refiner used for the deposit of nanoparticles of cerium and zirconium oxide is selected from the group of compounds comprising hydrofluoric, boric and carboxylic acids, and preferably formic, acetic, ascorbic and citric acids.
Still according to the invention, the pH of the solution is adjusted so as to allow the pickling of surface oxides from the metal substrate when it is in contact with the solution, so as to give the particles a maximum electrical charge in order to avoid any agglomeration in the solution and so as to make the particles as reactive as possible without destabilizing the solution.
In particular, the pH of the solutions based on nanoparticles of SiO2, SnO2, TiO2, ZnO or Sb2O5 is alkaline and is preferably between 9 and 13. The pH of the solutions based on nanoparticles of ZrO2, CeO2, SiO2 or Sb2O5 is acidic and is preferably between 1 and 5.
As an advantage, the pH of the solutions based on a mixture of nanoparticles is adjusted so that the solution is stable over time. Preferably, in the case of a surface layer of the substrate comprising a component of zinc, aluminum, iron, tin, chrome, nickel or copper, the pH is chosen to be either alkaline or acidic.
According to a first preferred embodiment of the invention, the deposit is achieved by immersing the substrate for a controlled period of time in an immersion tank containing the solution.
According to a second preferred embodiment of the invention, the deposit is achieved by spraying the solution onto the substrate by means of a nozzle, i.e. a device, assisted or not, with gas under pressure, that sprays droplets of the solution.
According to a third preferred embodiment of the invention, the deposit is created by depositing the solution on the substrate by means of a roller.
As an advantage, the solution that comes into contact with the strip is kept at a temperature of less than 100° C., and preferably less than 80° C.
As a further advantage, the temperature of the substrate at the start of the deposition is higher than 125° C. and lower than 250° C.
If the substrate already has a metallic coating before treatment, the temperature of the substrate at the start of the deposition is advantageously higher than 125° C. and lower by 30 to 100° C. than the melting point of the coating metal.
If the substrate has a metallic coating produced by immersion, as in galvanization by immersion, the deposition is preferably achieved just after the deposition of the metallic coating, before the substrate cools down.
Preferably, in the case of a substrate liable to a too-high level of oxidation for this to be eliminated during the deposition, the substrate is protected from excessive contact with air by means of a neutral gas such as nitrogen (N2) or argon.
Preferably again, the deposition is limited in time by varying the depth of immersion in the case of deposition in a solution or the length sprayed in the case of spraying the solution with nozzles.
Still according to the invention, the solution is an aqueous solution or comprises any other solvent capable of effectively dispersing said nanoparticles.
As an advantage, agents for the improvement of resistance to corrosion and/or adhesion to the substrate or the paint and/or to improve the glide during formation are added to the solution.
Provision can be made in the method of the invention for the coated substrate to be rinsed after post-treatment with water or with a solution based on organic silanes or carboxylic acid with an ability to form a strong link with the organic.
Preferably, the method of the invention comprises the means for:                continuously measuring and regulating the pH,        ensuring the replenishment of the solution and the elimination of surplus products of the reaction,        ensuring the homogeneous mixture of the bath so as to avoid turbulence on its surface.        
According to an advantageous embodiment, the temperatures of the strip and of the bath, the time the strip remains in the bath, the concentration of nanoparticles in the bath and the pH of the bath are controlled. If necessary, the temperature of the strip, the length of spraying time, the concentration of nanoparticles in the solution sprayed, the spraying flow and the pH are equally controlled.
A second aspect of the present invention relates to an installation for coating a steel strip, comprising a device for obtaining a second coating layer on a first coating layer obtained by hot dipping or by jet spraying, by implementing the above-described method, characterised in that said installation is located after elements ensuring the spinning and solidification operations of the first coating layer, said second coating layer being achieved in this installation at a temperature lower by at least 100° C. than the temperature at which the first coating layer solidifies.
A third aspect of the present invention relates to a flat or long metallurgical product, preferably a strip, wire, profiled section or tube, coated with an ultra-fine protective layer by means of the above-described method, characterised in that said protective layer comprises nanoparticles of oxide or of a mixture of these oxides, preferably Al2O3, Y2O3, SiO2, SnO2, TiO2, ZnO, Sb2O5, ZrO2 or CeO2, and has a thickness of less than 100 nm.
As an advantage, the invention relates to a metallurgical product of the strip coated type as described, the thickness of which, possibly the initial thickness before the profiled section or tube is produced, is between 0.15 and 5 mm.